FIG. 31 illustrates a hydraulic excavator 10, which is a work machine. The hydraulic exactor 10 includes a lower structure 11, an upper structure 12, a cab 13, work equipment 14 and a power system 15 that includes an engine. The cab 13, the work equipment 14 and the power system 15 are mounted on the upper structure 12, which is rotatably mounted on the lower structure 11. The power system 15 is covered by a top cover 16, side doors 17 and others. The side door 17 is mounted by a hinge 18 so as to be capable of opening and closing and secured in an opened state by means of a holding mechanism 19 illustrated in FIGS. 32 to 34.
Regarding the conventional holding mechanism 19 illustrated in FIGS. 32 to 34, when the side door 17 is closed, as illustrated in FIG. 33, the holding mechanism 19 is accommodated by hanging a stay (round bar) 19s folded and formed in a U-like shape between holes of brackets 19a, 19b additionally made on an upper structure frame 12f. When the side door 17 is opened, as illustrated in FIG. 34, the stay 19s is hooked by hanging the stay 19s between the hole of the bracket 19a additionally made on the upper structure frame 12f and the hole of a bracket 19d additionally made on the inner face of the side door 17 (for example, refer to Japanese Laid-open Patent Publication No. 9-315347 (pages 3 to 4, and FIGS. 1 to 4)).
In the above-described conventional holding mechanism 19, it is necessary to additionally install brackets 19a, 19b, 19d by welding for hanging the stay 19s on the upper structure frame 12f and the side door 17.
Further, the stay 19s is simply placed through the holes of the brackets 19a, 19b when the door is closed. Therefore, there is a concern that the stay 19s may come out of the holes of the brackets 19a, 19b and fall when the machine body is subjected to great vertical vibration. In order to prevent falling, as illustrated in FIGS. 32 to 34, a fall-prevention bracket 19e is additionally installed by welding on the inner face of the side door 17, thereby, when the door is closed, the stay 19s on the brackets 19a, 19b is to be pressed from above by the fall-prevention bracket 19e. 
Still further, in the conventional holding mechanism 19, when the side door 17 is opened, as illustrated in FIG. 34, the stay 19s is hooked by hanging the stay 19s between the holes of the brackets 19a, 19d. However, this movement is positioning work accompanied by attachment and detachment of the stay 19s, which is troublesome and may be omitted in a work field.
The present invention has been made in view of the above description, and an object of which is to provide a hinge and a door unit which are capable of securing a state where a door is opened without additionally installing brackets on a frame and a door member. Another object of the present invention is to provide a door unit capable of reliably and automatically securing a state where a door is opened with the hinge.